The present invention relates to the art of diagnostic imaging. It finds particular application in conjunction with multi-headed positron emission tomography (PET) scanners and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also applicable to combined computed tomography (CT) and SPECT scanners as well as other diagnostic modes in which nuclear detector heads may become saturated and/or damaged from impermissibly high levels of radiation.
Diagnostic nuclear imaging is used to study a radionuclide distribution in a subject. Typically, one or more radiopharmaceutical or radioisotopes are injected into a subject. The radiopharmaceuticals are commonly injected into the subject""s bloodstream for imaging the circulatory system or for imaging specific organs that absorb the injected radiopharmaceuticals. Sensitive scintillation crystal camera detector heads are placed adjacent to a surface of the subject to monitor and record emitted radiation. Typically, the detector heads are rotated or indexed around the subject in order to monitor the emitted radiation from a plurality of directions. In single photon emission computed tomography (SPECT), emission radiation is detected by one or more collimated detector heads. In positron emission tomography (PET), data collection is limited to emission radiation that is detected concurrently by a pair of oppositely disposed detector heads. The detected radiation data is then reconstructed into a three-dimensional image representation of the radiopharmaceutical distribution within the subject.
One of the problems with both PET and SPECT imaging techniques is that photon absorption and scatter by portions of the subject or subject support between the emitting radionuclide and the detector heads, distort the resultant image. In order to obtain more accurate SPECT and PET radiation attenuation measurements, a direct transmission radiation measurement is made using transmission computed tomography techniques. In the past, transmission radiation data was commonly acquired by placing a radioactive isotope line or point source opposite to a detector head, enabling the detector head to collect transmission data concurrently with the other two detector heads collecting emission data. This transmission data is then reconstructed into an image representation using conventional tomography algorithms. From this data, regional radiation attenuation properties of the subject, which are derived from the transmission computed tomography images, are used to correct or compensate for radiation attenuation in the emission data.
One PET scanning technique involves the injection of a radioisotope, which is selectively absorbed by tumors or other tissues of interest. The resulting PET images provide an accurate depiction of the location of the tumors in space. However, because only the radioactive isotope is imaged, the PET images provide no correlation between the image and the surrounding tissue. In order to coordinate the tumors with location in the patient, the same region of the subject is scanned with both the PET scanner and a computed tomography (CT) scanner. In the past, the PET and CT scanners were permanently mounted in a fixed relationship to each other. A patient was moved from one apparatus to the next. However, due to potential patient movement or repositioning between the CT scanner and the nuclear camera, this technique provided uncertainty in the alignment of the PET and CT images.
To eliminate the alignment problems associated with physically displaced imaging systems, it would be advantageous to mount the CT and nuclear imaging systems to a common gantry. However, nuclear detector heads are designed to detect very low levels of radiation. When exposed to higher levels of radiation, detector heads often saturate. The scintillation crystal on a detector head may be excited to such a high level that it continues to glow for an extended duration, which interferes with normal operation of the nuclear camera. In addition, very high radiation doses may even damage nuclear detector heads.
Although the x-rays of a CT scanner are intended to pass from the x-ray tube to the high energy x-ray detector, some of the x-rays are scattered in the patient or by scanner hardware. A significant number of radiation photons would find their way to the nuclear detector heads. Many of the gamma rays that reach the detector heads have been Compton scattered two, three, or more times in the patient. While these rays have lost significant amounts of energy, they are still well above the energy range of the detector head and may cause saturation of and/or damage to the detector heads.
The present invention contemplates a new and improved nuclear camera which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a diagnostic imaging system includes a stationary gantry which defines a subject receiving aperture and a source of penetrating radiation which transmits high energy radiation through a subject disposed in a subject receiving region. The radiation source is mounted for rotation around the stationary gantry subject receiving region. A radiation detector detects high energy radiation transmitted by the source after passage of the radiation through a subject in the subject receiving region. At least one nuclear detector head is mounted for rotation around the subject receiving region. The detector head detects low energy radiation emitted by a radiopharmaceutical injected into the subject. At least one reconstruction processor reconstructs high energy radiation received by the radiation detector and radiopharmaceutical radiation received by the nuclear detector head into volumetric image representations. A fusion processor combines the high energy and radiopharmaceutical radiation volumetric image representations together. A shield shields the nuclear detector heads from the high energy radiation.
In accordance with a more limited aspect of the present invention, each nuclear detector head includes a scintillation crystal which emits a short duration light scintillation in. response to radiopharmaceutical radiation incident thereon and which glows emitting light for a longer duration in response to scattered high energy radiation. A plurality of opto-electrical elements are optically coupled to the scintillation crystal. The opto-electrical elements convert light received from the scintillation into a plurality of electrical output signals. A variable axial radiation shield is disposed adjacent the scintillator. The variable axial radiation shield shields the scintillation crystal from at least one of non-axial radiation events originating from the injected radiopharmaceutical and the high energy radiation originating from the source of penetrating radiation.
In accordance with a more limited aspect of the present invention, the variable axial radiation shield includes a plurality of substantially parallel vanes movably mounted adjacent the scintillation crystal.
In accordance with a more limited aspect of the present invention, the variable axial radiation shield includes a means for pivoting the plurality of substantially parallel vanes from an open orientation, which is substantially perpendicular to the scintillation crystal, to a closed orientation, which blocks radiation from reaching the scintillation crystal.
In accordance with another aspect of the present invention, a diagnostic imaging system includes a rotating gantry which defines a subject receiving aperture and a source of penetrating radiation and a corresponding detector means for generating a computed tomographic image representation of a subject disposed within the subject receiving aperture. A plurality of nuclear detector heads are rotatably mounted to the gantry, each detector head having a radiation receiving face and a variable radiation filter for selectively restricting and permitting radiation to strike the radiation receiving face. The variable radiation filter includes a plurality of vanes movably mounted across the radiation receiving face. In the diagnostic imaging system, a method of diagnostic imaging includes positioning the plurality of vanes of the variable radiation filter such that they block radiation from striking the radiation receiving face. Radiation from the radiation source is transmitted through the subject and toward the corresponding detector means positioned across the receiving subject aperture. The transmitted radiation is reconstructed into a volumetric image representation. A radiopharmaceutical is injected into the subject disposed within the subject receiving aperture. The plurality of vanes of the variable radiation filter are positioned such that radiation emitted by the radiopharmaceutical is receivable by the radiation receiving face. Radiation emitted by the radiopharmaceutical is detected and reconstructed with an emission image representation. The reconstructed volumetric and emission image representations are combined into a combined image representation.
In accordance with another aspect of the present invention, a detector head for use in a nuclear camera includes a scintillator which emits light in response to incident radiation. A plurality of opto-electrical elements, which are optically coupled to the scintillator, convert light received from the scintillator into a plurality of electrical output signals. A variable axial radiation shield, which is disposed adjacent the scintillator, is moveable between (i) an open configuration in which it collimates incident radiation and (ii) a closed configuration in which it blocks incident radiation from reaching the scintillator.
In accordance with a more limited aspect of the present invention, the variable axial radiation shield includes a plurality of vanes tiltably mounted adjacent the scintillator.
In accordance with a more limited aspect of the present invention, the variable axial radiation shield includes a means for tilting the plurality of vanes between at least an orientation substantially perpendicular to the scintillator in the open configuration and an orientation substantially parallel to the scintillator in the closed configuration.
One advantage of the present invention resides in the elimination of detector saturation due to scattered radiation from external radiation sources.
Another advantage of the present invention is that it facilitates combined CT/PET diagnostic imaging.
Another advantage of the present invention resides in the simplicity and ease of use.
Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.